1. Field of the Invention
The present invention relates to an apparatus and method for Transmission Time Interval (TTI) reconfiguration in a mobile communication system. More particularly, the present invention relates to an apparatus and method for, upon change of a TTI, reducing a loss of data while performing Hybrid Automatic Repeat reQuest (HARQ) retransmission in a system having two or more TTIs such as a High-Speed Uplink Packet Access (HSUPA) system.
2. Description of the Related Art
Recent wireless mobile communication systems are in the trend of demand for a high-speed packet service. Thus, a high-speed packet is being standardized for an uplink as well as a downlink even in an existing Wideband Code Division Multiple Access (WCDMA) system. High-Speed Uplink Packet Access (HSUPA) system uses an Enhanced Dedicated CHannel (E-DCH) added to an uplink of a WCDMA system, as a channel for high-speed packet. The HSUPA system can perform high-speed data transmission at high reliability because supporting an uplink Hybrid Automatic Repeat reQuest (HARQ). A HARQ scheme used is an N-channel Stop And Wait HARQ (N-channel SAW HARQ).
The N-channel SAW HARQ is described in detail below. The N-channel SAW HARQ is a scheme newly introducing two schemes below to enhance the efficiency of a general SAW ARQ scheme.
The first scheme is a scheme in which a reception side temporarily stores erroneous data and combines the erroneous data with a retransmission of corresponding data, thus reducing a probability of error occurrence. To temporarily store erroneous data and combines the erroneous data with a retransmission is called “soft combining” The soft combining is divided into Chase Combining (CC) and Incremental Redundancy (IR) techniques.
In the CC technique, a transmission side uses the same transport format for initial transmission and retransmission. If symbols of ‘m’ number are transmitted as one coded block during initial transmission, symbols of the same ‘m’ number are transmitted during retransmission as well. That is, the initial transmission and retransmission apply the same coding rate. A reception side combines an initially transmitted data block with a retransmitted data block, performs Cyclic Redundancy Code (CRC) operation using the combined data block, and identifies whether an error occurs.
In the IR technique, a transmission side uses a different transport format for initial transmission and retransmission. If user data of ‘n’ bits are converted into symbols of ‘m’ number through channel coding, the transmission side transmits only part of the symbols of the ‘m’ number during initial transmission, and sequentially transmits remaining parts, which have not been transmitted during initial transmission, during retransmission. That is, the initial transmission and retransmission have a different coding rate. A reception side configures a data block having a high coding rate by attaching a retransmitted data block to the rear of an initially transmitted data block and then, executes error correction. In the IR technique, the initially transmitted data block and retransmitted data block are distinguished using a version number. Initial transmission is named as ‘Version 1’, next retransmission is named as ‘Version 2’, and further next retransmission is named as ‘Version 3’. The reception side can correctly combine an initially transmitted data block with a retransmitted data block using version information.
The second scheme, newly introduced into the N-channel SAW HARQ to enhance the general SAW ARQ scheme, is described below. The general SAW ARQ scheme cannot transmit a next packet until receiving ACKnowledgment (ACK) of a previous packet. Unlike this, the N-channel SAW HARQ continuously transmits a plurality of packets with not receiving ACK, thus being capable of enhancing the efficiency of use of a radio link. If the N-channel SAW HARQ sets logical channels of ‘N’ number between a User Equipment (UE) and a NodeB, and a reception side identifies the logical channels by a specific time or explicit channel number, the reception side can be aware if a packet received at any time point belongs to any channel, and can independently perform HARQ process of reconfiguring packets in sequence having to be received or processing a corresponding packet by soft combining
An uplink HARQ process is described below. In a HSUPA system, an E-DCH channel supports a 10 msec TTI and a 2 msec TTI. Number of HARQ processes for the 10 msec TTI is equal to 4, and number of HARQ processes for the 2 msec TTI is equal to 8. The number of HARQ processes is decided depending on a round trip delay time between a UE and a Node B.
FIGS. 1A and 1B are a diagram illustrating a HARQ process for each TTI in an E-DCH channel of a HSUPA system according to the conventional art. FIG. 1A is a diagram illustrating a HARQ process for a 10 msec TTI. FIG. 1B is a diagram illustrating a HARQ process for a 2 msec TTI.
Referring to FIG. 1A, a UE transmits 120 data received from an upper layer, to a Node B over an E-DCH channel in a process 1 100. The Node B decodes the received data, performs a Cyclic Redundancy Code (CRC) check to determine if the data is normal data or abnormal data, and transmits 121 an ACK signal for normal data or a Non ACKnowledgment (NACK) signal for abnormal data to the UE over an E-DCH HARQ acknowledgment Indicator CHannel (E-HICH). Upon receiving the ACK signal, the UE transmits new data to the Node B. Upon receiving the NACK signal, the UE retransmits data having been transmitted in the previous process 1 100, in a next process 1 104. Such retransmission is repeated till an ACK signal is forwarded, or is repeated by the maximum number of times of retransmission. The aforementioned operation is identically applied to a process 2 101, a process 3 102, and a process 4 103 for a 10 msec TTI. FIG. 1B shows the same operation as that of FIG. 1A, but eight HARQ processes operate sequentially.
Unlike a High-Speed Downlink Packet Access (HSDPA) system supporting only a short 2 msec TTI, a HSUPA system is specified in the 3rd Generation Partnership Project (3GPP) Release 7 standard to simultaneously support both a 10 msec TTI and a 2 msec TTI depending on UE's capacity. Also, a High-Speed Dedicated CHannel (HS-DCH) of the HSDPA system supports a hard handoff, while an E-DCH of the HSUPA system supports a soft handover.
As described above, an E-DCH for high-speed data transmission supports both a 10 msec TTI and a 2 msec TTI in a HSUPA system. The HSUPA system uses a 2 msec TTI to increase a transmission speed and reduce a delay time. Depending on a communication environment and a system environment, a 10 msec TTI is used and then reconfigured as a 2 msec TTI, or a 2 msec TTI is used and then reconfigured as a 10 msec TTI. If TTI reconfiguration is performed with a HARQ process not fully terminated, data remaining in a buffer is deleted and is not transmitted. Such packet data can be restored through ARQ at RLC level, but this deteriorates the efficiency of HARQ.
In order to solve the problem, the conventional art discloses that, if a command of TTI reconfiguration is issued, moment wait is done and then, TTI reconfiguration is performed after a previous HARQ process is all terminated. However, the conventional art has a disadvantage that, when a command of TTI reconfiguration is issued from a NodeB, it takes a long delay time until a HARQ process is terminated. Also, the conventional art has a problem that a delay of a change of TTI may lead to a call-drop phenomenon, because there is a strong possibility of changing a 2 msec TTI into a 10 msec TTI when a channel environment is bad or a signal intensity is weak.